I'm not sure what you are trying to achieve with the media & photons. Unless
they are specifically adding something to the scene I would remove them
since it won't work as a bugfix.

I tried your scene file, and two things that worked were:

1.  Add the 'hollow' keyword just before the pigment definition.  Because
the box has zero thickness, povray may be hitting the back wall before it
hits the front wall, and then things get confusing.  Or mabye it thinks
it's still inside the box when it isn't.

OR

2.  Give the box a very small thickness, say 0.001.

Either approach seemed to work, I can't be sure of the total effect because
I don't have your PNG file.

I think the problem stems from using a zero thickness solid object, instead
of a one-sided object.  You could try using a union of two triangles, or a
polygon object, or a disc, in place of the box.

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